I've jacked up the beam and re-supported it with adjustable jack-posts and all seems well except that the joists don't seem to have moved with the beam. In other words, the beam is far more level and the floor directly above the beam is just as level, but the floors simply have a "speed bump" shape in them where the beam is supporting them but they quickly sag off to either side of the beam.
The joists appear to be in good condition, though they're certainly original and they are supported both by being mortised into the beam and additionally by bulky (1/4" thick?) 90-degree angle braces. I'm trying to figure out why the beam moved but the joists apparently did not.
N.B. - I did jack the beam up relatively quickly. Over the course of maybe a month. Could that have something to do with it? Help appreciated, thanks.